Fix potential race of getting caps during addNet

getmtu can return IOError when a libvirt network underlying device
does not exist. This can happen when we get a getVdsCapabilities
request after the libvirt network entry has been added but before
the device has been ifupped.

Change-Id: Idfabbc7799b330412e864dfb6f77748e57f7df85
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/16122
Reviewed-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>